Laterally fluorinated 4-cyanophenyl and 4&#39;-cyanobiphenyl benzoates

ABSTRACT

Laterally fluorinated 4-cyanophenyl and 4&#39;-cyanobiphenyl benzoates are  diosed of general formula I: ##STR1## in which R is R 1  or R 1  O, wherein R 1  is C 1  -C 12  alkyl, m and n are 0 or 1 provided that (n+m) is 0 or 1, each of a, b, c and d are independently 0 or 1, provided (a+b+c+d) is not zero, but excluding the case where a and b are 0 and one of c and d is 0, being useful constituents of nematic liquid crystal materials.

This invention relates to laterally fluorinated 4-cyanophenyl and4'-cyanobiphenyl benzoates, and to liquid crystal mixtures and devicesusing them.

Liquid crystal materials are well known for use in electroopticaldisplay devices such as calculators, watches and the like. A commonlyused type of liquid crystal material is that which shows a nematicliquid crystalline phase. Desirable characteristics in a nematic liquidcrystal material include a high nematic to isotropic (N-I) transitiontemperature, a low solid crystal to nematic (K-I) transitiontemperature, and the absence of smectic (S) phases. Other desirablecharacteristics include low viscosity, ease of manufacture, chemical andthermal stability.

Nematic liquid crystal materials generally consist of a mixture ofchemical compounds. One class of such compounds are 4-cyanophenyl and4'cyanobiphenyl esters of structure: ##STR2## where R may be alkyl,alkoxy, acyloxy or alkylcarbonato disclosed respectively in U.S. Pat.Nos. 4,138,359 and 3,951,846. Some halogenated analogues of thesecompounds have been investigated, for example fluorinated compounds suchas: ##STR3## are described in Mol Cryst Liq Cryst 109(2-4), (1984) (

169-78 and in Japanese Patent Applications 58/210982, 60/69190,63/122669, 61/30565 and 61/50953, and German Offenlegungschrift 3339216.

JPA 61/106550 describes compounds of structure: ##STR4##

JPA 61/63645 describes the compound: ##STR5##

U.S. Pat. No. 4,689,176 discloses optically active compounds of formula:##STR6## where R is alkyl, 1 is 1 or 2 and m is 0 or 1.

To date, the extent to which fluorinated compounds for use as liquidcrystal compounds could be investigated has been hampered by theunavailability of suitable starting materials, and the difficulties oforganofluorine chemistry.

The present inventors have investigated the field of fluorinated4-cyanophenyl and 4'-cyanobiphenyl esters and have identified a range ofnovel compounds which are useful components of liquid crystal materials.In the course of this a number of novel and potentially usefulfluorinated intermediates have also been prepared.

According to the present invention, novel compounds are provided, havinga formula I: ##STR7## in which R is R¹ or R¹ O, wherein R¹ is C₁ -C₁₂alkyl, n and m are 0 or 1 provided that (n+m) is 0 or 1, each of a, b, cand d are independently 0 or 1, provided (a+b+c+d) is not zero, butexcluding the case where a and b are 0 and one of c or d is one.

Compounds of formula I are useful as constituents of liquid crystalcompositions, particularly nematic liquid crystal materials. Thestructural preferences expressed below are based inter alia on relativeusefulness in nematic liquid crystal materials.

Preferably R¹ contains 3 to 8 carbon atoms, especially 3, 4 or 5, and R¹is n-alkyl or an asymmetrically substituted alkyl group such as2-methylbutyl which may be in an optically active or racemic form. It isparticularly preferred that R is n-alkyl or n-alkoxy.

Some preferred overall structures for compounds of Formula I are listedin table 1 below.

                  TABLE 1                                                         ______________________________________                                         ##STR8##                     1.1                                              ##STR9##                     1.2                                              ##STR10##                    1.3                                              ##STR11##                    1.4                                              ##STR12##                    1.5                                              ##STR13##                    1.6                                              ##STR14##                    1.7                                              ##STR15##                    1.8                                              ##STR16##                    1.9                                              ##STR17##                    1.10                                             ##STR18##                    1.11                                             ##STR19##                    1.12                                             ##STR20##                    1.13                                             ##STR21##                    1.14                                            ______________________________________                                    

Of the structural types listed in table 1, those where R is n-alkyl orn-alkoxy and which contain three phenyl rings generally have higher K-Nand N-I transistions than their two-ringed counterparts, whereas thelatter often show nematic phases which supercool to well below roomtemperature. Both of these characteristics are of value in liquidcrystal compositions.

Compounds of formula I, including those listed in table 1 may beprepared by a number of routes. Being phenylzoate esters they may forexample be prepared from a phenol and benzoic acid carrying fluorine andother substituents R and CN in appropriate positions. Esterification maybe via the dicyclohexylcarbodiimide (DCC) mediated method, the reactionand purification conditions of which are well known to those skilled inthe art.

Some fluoro-substituted phenols and benzoic acids are commericallyavailable, such as 3-fluoro-4-cyanophenol, but others may be prepared bysynthetic routes.

Suitable fluoro-substituted phenols may be prepared by schemes A and Dshown in FIGS. 1 and 2, and suitable fluoro-substituted benzoic acidsmay be prepared by schemes B and C shown in FIGS. 3 and 4. The steps ofthese routes are summarised below: ##STR22##

Known acids which may be used in the esterification reaction includenon-fluorinated 4-alkyl and 4-alkoxy benzoic acids, and 4-alkyl and4-alkoxy phenyl-4'-benzoic acids. These may be used to preparestructures 1.1 and 1.8 listed in table 1 above and other compounds offormula I in which a and b are both O, and n is 0 or 1.

Compounds of formula I are suitable components of liquid crystalmaterials, particularly nematic liquid crystal materials.

According to a further aspect of the invention there is thereforeprovided a liquid crystal material, being a mixture of at least twocompounds, at least one of which is a compound of formula I.

The material may contain two or more compounds of formula I, for examplediffering only in their substituent R, eg two different n-alkyl and/orn-alkoxy groups. Preferably the materials according to this aspect ofthe invention contains one or more compounds having a high dielectricanisotropy, and selected from table 2 below:

                  TABLE 2                                                         ______________________________________                                         ##STR23##                     2.1                                             ##STR24##                     2.2                                             ##STR25##                     2.3                                             ##STR26##                     2.4                                             ##STR27##                     2.5                                             ##STR28##                     2.6                                             ##STR29##                     2.7                                             ##STR30##                     2.8                                            ______________________________________                                    

where R² is alkyl or alkoxy containing 3 to 10 carbon atoms. It isespecially preferred that the material contains one or more compounds offormula 2.1, optionally together with one or more compounds of formula2.8.

The liquid crystal material of this aspect of the invention may alsocontain compounds of low dielectric anisotropy, for example one or morecompounds as listed below in table 3:

                  TABLE 3                                                         ______________________________________                                         ##STR31##                     3.1                                             ##STR32##                     3.2                                             ##STR33##                     3.3                                             ##STR34##                     3.4                                             ##STR35##                     3.5                                             ##STR36##                     3.6                                             ##STR37##                     3.7                                             ##STR38##                     3.8                                            ______________________________________                                    

Where R³ and R⁴ are independently C₃ to C₁₀ alkyl or alkoxy and X isfluorine or hydrogen.

Typically but not exclusively a liquid crystal material of this aspectof the invention will contain:

    ______________________________________                                         high dielectric anisotropy compound(s)                                                                    50-95 wt %                                       e.g. as listed in table 2                                                      low dielectric anisotropy compound(s)                                                                     0-50 wt %                                        e.g. as listed in table 3                                                     compound(s) of formula I     5-50 wt %                                        additives                    0-10 wt %                                        ______________________________________                                    

the total being 100 wt %.

Additives may for example include pleochroic dyes, e.g. one or more ofthose described in EP-A-82300891.

The liquid crystal material of this aspect of the invention may be usedin any of the known types of nematic liquid crystal device, for example:the twisted nematic effect device, which may be multiplexed; theFreedericksz effect device using materials of positive or negativedielectric anisotropy; the cholesteric memory mode device; thecholesteric to nematic phase change effect device (these latter twodevices require a material which contains one or more optically activecompounds, for example a compound of formula I, or a compound as listedin table 2 or 3 in which R, R², R³ or R⁴ is a group containing anasymmetrically substituted carbon atom, in particular (+) or (-)2-methylbutyl or 2-methylbutyloxy), the dynamic scattering effect deviceor the two frequency switching effect device. A liquid crystalelectrooptic display device which uses a liquid crystal material that isa mixture of at least two compounds, one of which is a compound offormula I is a further aspect of this invention. The method ofconstruction and operation of such devices is well known in the art.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of example only withreference to:

FIGS. 1, 2, 3 and 4 showing preparative rotes to fluorinated phenols andbenzoic acids.

FIGS. 5 and 6 showing routes to compounds of formula I.

FIG. 7 showing a liquid crystal electro-optic display device.

EXAMPLE 1 Scheme A 4-Methoxyphenylboronic acid (1) Step 1A

A solution of the Grignard reagent, prepared from 4-bromoanisole (80.0g, 0.43 mol) and magnesium (11.96 g, 0.49 mol) in dry THF (300 ml) wasadded dropwise to a stirred, cooled (-78° C.) solution of tri-isopropylborate (161.7 g, 0.86 mol) in dry THF (50 ml) under dry nitrogen. Thestirred mixture was allowed to warm to room temperature overnight andwas then stirred at room temperature for 1 h with 10% hydrochloric acid(300 ml). The product was extracted into ether (twice), the combinedethereal extracts were washed with water and dried (MgSO₄). The solventwas removed in vacuo to yield a cream-coloured solid (58.5 g) which wasrecrystallised from water to give colourless crystals.

Yield: 27.7 g, 43%.

mp: 201°-202° C.

Note: A cream-coloured oil was separated during the aboverecrystallisation which solidified on cooling to give a cream-colouredsolid.

Yield: 17.5 g.

4-Bromo-2-fluoroaniline (2) Step 2A

N-Bromosuccinimide (160.4 g, 0.90 mol) was added in small portions to astirred, cooled solution of 2-fluoroaniline (100.0 g, 0.90 mol) in drydichloromethane (400 ml). The stirred mixture was allowed to warm to 0°C. over 45 min, washed with water and dried (MgSO₄). The solvent wasremoved in vacuo to yield a red solid (180 g) which was steam distilledto give a colourless solid.

Yield: 120.0 g, 70%.

mp: 41°-42° C.

4-Bromo-2-fluorobenzonitrile (3) Step 4A

Concentrated sulphuric acid (60 ml) was added dropwise to a stirredmixture of compound 2 (75.0 g, 0.39 mol), water (150 ml) and glacialacetic acid (185 ml). The clear solution was cooled to -5° C. (anemulsion formed) and a solution of sodium nitrite (30.0 g, 0.44 mol) inwater (100 ml) was added dropwise; the stirred mixture was maintained at-5° C. for 15 min.

A solution of potassium cyanide (128.3 g, 1.98 mol) in water (300 ml)was added dropwise to a stirred solution of copper(II) sulphatepentahydrate (118.1 g, 0.48 mol) in water (300 ml) and ice (300 g) at10°-20° C. Sodium hydrogen carbonate (265.1 g, 3.16 mol) and cyclohexane(450 ml) were added, the temperature was raised to 50° C. and the colddiazonium salt mixture was added in portions with rapid stirring. Themixture was cooled, the organic layer was separated and the aqueouslayer was washed with ether (twice). The combined organic phases werewashed with water, 10% sodium hydroxide, water and dried (MgSO₄). Thesolvent was removed in vacuo to afford a dark brown solid (78.0 g) whichwas steam distilled to give an off-white solid.

Yield: 59.2 g, 76%.

mp: 69°-70° C.

4-Cyano-3-fluoro-4'-methoxybiphenyl (4) Step 5A

A solution of compound 1 (10.64 g, 0.07 mol) in ethanol (90 ml) wasadded dropwise to a stirred mixture of compound 3 (12.0 g, 0.06 mol) andtetrakis(triphenylphosphine)palladium(O) (2.10 g, 1.82 mmol) in benzene(125 ml) and 2M-sodium carbonate (100 ml) at room temperature under drynitrogen. The stirred mixture was heated under reflux (90°-95° C.) for2.5 h (i.e., until glc analysis revealed absence of starting material).The mixture was cooled and stirred for 1 h at room temperature with 30%hydrogen peroxide (2 ml). The mixture was cooled further to 2° C., theproduct was filtered off and washed with water. The product was dried(CaCl₂) in vacuo to give a yellow powder.

Yield: 11.9 g, 88%.

mp: 153°-154° C.

4-Cyano-3-fluoro-4'-hydroxybiphenyl (5) Step 6A

A solution of boron tribromide (12.0 ml, 31.8 g, 0.13 mol) in drydichloromethane (100 ml) was added dropwise to a stirred, cooled (-78°C.) solution of compound 4 (14.0 g, 0.06 mol) in dry dichloromethane(250 ml) under dry nitrogen. The stirred mixture was allowed to warm. toroom temperature over 20 h (i.e., until glc analysis revealed absence ofstarting material). Water was added and a yellow precipitate wasproduced. The product was extracted into ether (twice), and the combinedethereal extracts were washed with water and dried (MgSO₄). The solventwas removed in vacuo to afford a pale-yellow powder.

Yield: 13.1 g, 100%.

mp: 201°-202° C.

4-Bromo-2,6-difluoroaniline (6) (Step 3A)

A solution of bromine (127.0 g, 0.79 mol) in glacial acetic acid (200ml) was added slowly, dropwise to a stirred solution of2,6-difluoroaniline (101.5 g, 0.79 mol) in glacial acetic acid (550 ml)keeping the temperature below 25° C. The mixture was stirred at roomtemperature for 2 h, and then sodium thiosulphate (50 g), sodium acetate(125 g) and water (700 ml) were added and the mixture was cooled in arefrigerator overnight. The product was filtered off, dissolved inether, and the ether phase was washed with water, 10% sodium hydroxide,water and dried (MgSO₄). The solvent was removed in vacuo to yield apale yellow solid (134 g) which was steam distilled to give a colourlesssolid.

Yield: 117.9 g, 72%.

mp: 67°-68° C.

4-Bromo-2,6-difluorobenzonitrile (7) Step 4A

A stirred mixture of compound 6 (56.0 g, 0.27 mol) and concentratedsulphuric acid-water (1:1, 250 ml) was heated until a solution wasobtained. The mixture was cooled to -10° C. and a solution of sodiumnitrite (30.9 g, 0.45 mol) in water (70 ml) was added dropwise. Themixture was stirred at -5° C. for 2.5 h.

A solution of potassium cyanide (97.5 g, 1.50 mol) in water was addeddropwise to a stirred solution of copper(II) sulphate pentahydrate (86.5g, 0.35 mol) in water (200 ml) and ice (130 g). Sodium hydrogencarbonate (300 g), cyclohexane (1200 ml) and glacial acetic acid (130ml) were added, the temperature was raised to 50° C. and the colddiazonium salt mixture was added in portions with rapid stirring. Themixture was cooled, the organic layer was separated and the aqueouslayer was washed with ether (twice). The combined organic phases werewashed with water, 10% sodium hydroxide, water and dried (MgSO₄). Thesolvent was removed in vacuo to afford a dark-brown solid (58.5 g) whichwas steam distilled to give a light-orange solid.

Yield: 31.7 g, 54%.

mp: 79°-80° C.

4-Cyano-3,5-difluoro-4'-methoxybiphenyl (8) Step 5A

A solution of compound 1 (11.50 g, 0.076 mol) in ethanol (90 ml) wasadded dropwise to a stirred mixture of compound 7 (15.0 g, 0.069 mol)and tetrakis(triphenylphosphine)palladium(O) (2.38 g, 2.06 mmol) inbenzene (130 ml) and 2M-sodium carbonate (100 ml) at room temperatureunder dry nitrogen. The stirred mixture was heated under reflux (90°-95°C.) for 4.5 h (i.e., until glc analysis revealed absence of startingmaterial). The mixture was cooled and stirred for 1 h at roomtemperature with 30% hydrogen peroxide (2 ml). The mixture was furthercooled to 2° C., and the product was filtered off and washed with water.The product was dried (CaCl₂) in vacuo to give fawn-coloured needles.

Yield: 14.1 g, 84%.

mp: 205°-207° C.

4-Cyano-3,5-difluoro-4'-hydroxybiphenyl (9) Step 6A

A solution of boron tribromide (25.0 ml, 61.5 g, 0.25 mol) in drydichloromethane (140 ml) was added dropwise to a stirred, cooled (-78°C.) solution of compound 8 (12.22 g, 0.05 mol) in dry dichloromethane(250 ml) under dry nitrogen. The stirred mixture was allowed to warm toroom temperature over 9 h (i.e., until glc analysis revealed absence ofstarting material). The mixture was cooled to -10° C., water was addedslowly (exothermic) and cooled to 2° C. The product was filtered off,washed with water and dried (CaCl₂) in vacuo to give a pale-yellowpowder.

Yield: 13.3 g, 100%.

mp: 230°-231° C.

EXAMPLE 2 Scheme D 2,6-Difluoro-4-methoxybenzoic acid (24) Step 1D

A solution of n-butyllithium (10.0M in hexane; 25.0 ml, 0.25 mol) wasadded dropwise to a stirred, cooled (-78° C.) solution of3,5-difluoroanisole (35.9 g, 0.25 mol) in dry THF (200 ml) under drynitrogen. The stirred mixture was maintained under these conditions for2.5 h and then poured into a slurry of solid carbon dioxide and dryether. The product was extracted into 10% sodium hydroxide (twice), thecombined basic extracts were acidified with 36% hydrochloric acid, andthe product was extracted into ether (twice), the combined etherealextracts were washed with water and dried (MgSO₄). The solvent wasremoved in vacuo to give a colourless solid.

Yield: 46.5 g, 99%.

mp: 184°-185° C.

2,6-Difluoro-4-methoxybenzoyl chloride (25) Step 2D

A solution of oxalyl chloride (36.5 g, 0.29 mol) in dry benzene (50 ml)was added dropwise to a stirred solution of compound 24 (25.5 g, 0.14mol) and dry DMF (40 drops) in dry benzene (500 ml) at room temperature.The mixture was stirred at room temperature overnight and the excess ofoxalyl chloride and benzene were removed in vacuo.

2,6-Difluoro-4-methoxybenzamide (26) Step 3D

The crude acid chloride (25) was dissolved in dry diglyme (60 ml) andadded dropwise at room temperature with gentle stirring to 35% ammonia(750 ml). The resulting colourless precipitate was filtered off anddried (CaCl₂) in vacuo to give a colourless solid.

Yield: 21.2 g, 81%.

mp: 158°-159° C.

2,6-Difluoro-4-methoxybenzonitrile (27) Step 4D

A solution of thionyl chloride (139.0 g, 1.17 mol) in dry DMF (150 ml)was added to a stirred solution of compound 26 (20.0 g, 0.11 mol) in dryDMF (250 ml) at room temperature. The mixture was stirred at roomtemperature overnight and then poured onto ice-water. The product wasextracted into ether (twice), and the combined ethereal extracts werewashed with water, saturated sodium hydrogen carbonate, water and dried(MgSO₄). The solvent was removed in vacuo to give off-white crystals.

Yield: 17.7 g, 95%.

mp: 59°-60° C.

2,6-Difluoro-4-hydroxybenzonitrile (28) Step 5D

A stirred, homogeneous mixture of finely powdered, compound 27 (17.0 g,0.10 mol), aluminium chloride (29.4 g, 0.22 mol) and sodium chloride(7.1 g, 0.12 mol) was heated to 180° C. over 25 min and then at 180° C.for 1 h (glc and tlc analyses both revealed a complete reaction).Ice-water was added, and the product was extracted into ether (twice).The combined ethereal extracts were washed with water, and the productwas extracted into 10% sodium hydroxide (twice) and the combined basicextracts were acidified with 36% hydrochloric acid. The product wasextracted into ether (twice), and the combined ethereal extracts werewashed with water and dried (MgSO₄). The solvent was removed in vacuo togive a fawn solid.

Yield: 14.6 g, 94%.

mp: 119°-120° C.

EXAMPLE 3 Scheme B 1-(3,5-Difluorophenyl)pentan-1-ol (11) Step 1B

A solution of pentanal (19.0 g, 0.22 mol) in dry ether (75 ml) was addeddropwise to a stirred solution of the Grignard reagent at roomtemperature, under dry nitrogen, prepared from1-bromo-3,5-difluorobenzene (50.2 g, 0.26 mol) and magnesium (7.25 g,0.30 mol) in dry THF (100 ml). The stirred mixture was heated underreflux for 2 h, cooled and a saturated solution of ammonium chloride(300 ml) was added. The product was extracted into ether (twice), andthe combined ethereal extracts were washed with water and dried (MgSO₄).The solvent was removed in vacuo to give a pale-orange liquid (51.1 g)which was distilled to yield a colourless liquid. Smaller quantities ofcrude product were purified by column chromatography (silcagel/dichloromethane) to yield a colourless liquid (92%).

Yield: 35.8 g, 69%.

bp: 90° C. at 0.5 mm Hg.

3,5-Difluoro-1-pent-1'-enylbenzene (12) Step 2B

Phosphorus(V) oxide (64.5 g, 0.45 mol) was added in portions to astirred solution of compound 11 (35.0 g, 0.175 mol) in dry pentane (150ml) at room temperature. The mixture was stirred at room temperatureovernight (glc analysis revealed absence of starting material) andfiltered.

3,5-Difluoro-1-pentylbenzene (13) Step 3B

5% Palladium-on-charcoal (4.0 g) was added to the above filtrate [i.e.compound 12 in pentane (150 ml)] at room temperature. The stirredmixture was hydrogenated at atmospheric pressure for 4 h (i.e., untilglc analysis revealed absence of starting material) and the mixture wasfiltered. The majority of the pentane was removed in vacuo, and thefinal amount of pentane was distilled off (760 mm Hg) at 38° C. and theresidue was then distilled to yield a colourless liquid.

Yield: 23.4 g, 73%.

bp: 200° C. at 760 mm Hg.

2,6-Difluoro-4-pentylbenzoic acid (14) Step 4B

A solution of n-butyllithium (10.5M in hexane; 2.70 ml, 0.028 mol) wasadded dropwise to a stirred, cooled (-78° C.) solution of compound 13(5.10 g, 0.028 mol) in dry THF (30 ml) under dry nitrogen. The stirredmixture was maintained under these conditions for 2.5 h and then pouredonto a slurry of solid carbon dioxide and dry ether. The product wasextracted into 10% sodium hydroxide which was then acidified with 36%hydrochloric acid. The acidic mixture was washed with ether (twice), andthe combined ethereal layers were washed with water and dried (MgSO₄).The solvent was removed in vacuo to yield a colourless solid.

Yield: 5.8 g, 91%.

mp: 75°-76° C.

2,6-Difluoro-4-pentylphenylboronic acid (15) Step 5B

A solution of n-butyllithium (10.4M in hexane; 2.70 ml, 0.028 mol) wasadded dropwise to a stirred, cooled (-78° C.) solution of compound 13(5.00 g, 0.027 mol) in dry THF (60 ml) under dry nitrogen. The stirredmixture was maintained under these conditions for 2.5 h and a previouslycooled solution of tri-isopropyl borate (10.22 g, 0.054 mol) in dry THF(50 ml) was added dropwise at -78° C. The stirred mixture was allowed towarm to room temperature overnight and stirred for 1 h with 10%hydrochloric acid (30 ml). The product was extracted into ether (twice),and the combined ethereal extracts were washed with water and dried(MgSO₄). The solvent was removed in vacuo to give a colourless solid.

Yield: 6.7 g, 100%.

mp: 95°-100° C.

1-Butoxy-3,5-difluorobenzene (16) Step 6B

A solution of 1-bromobutane (11.0 g, 0.08 mol) in acetone (30 ml) wasadded dropwise to a stirred, refluxing mixture of 3,5-difluorophenol(9.1 g, 0.07 mol) and potassium carbonate (30 g, 0.2 mol) in acetone(200 ml). The stirred mixture was heated under reflux for 24 h (glcanalysis revealed absence of starting material). The product wasextracted into ether (twice), and the combined ethereal extracts werewashed with water, 5% sodium hydroxide, water and dried (MgSO₄). Most ofthe solvent was removed in vacuo, and after the removal of the finalamount of solvent and the excess of 1-bromobutane at atmosphericpressure, the residue was distilled to yield a colourless liquid.

Yield: 11.7 g, 90%.

bp: 204° C. at 760 mm Hg.

4-Butoxy-2,6-difluorobenzoic acid (17) Step 4B

A solution of n-butyllithium (10.5M in hexane; 3.60 ml, 0.038 mol) wasadded dropwise to a stirred, cooled (-78° C.) solution of compound 16(7.00 g, 0.038 mol) in dry THF (35 ml) under dry nitrogen. The stirredmixture was maintained under these conditions for 2.5 h and then pouredonto a slurry of solid carbon dioxide and dry ether. The product wasextracted into 10% sodium hydroxide which was then acidified with 36%hydrochloric acid. The product was filtered off and dried (CaCl₂) invacuo to give a colourless solid.

Yield: 8.3 g, 96%.

mp: 110°-112° C.

EXAMPLE 4 Scheme C 4-Butoxy-2-fluorobenzonitrile (18) Step 4C

A solution of 1-bromobutane (18.0 g, 0.13 mol) in acetone (30 ml) wasadded dropwise to a stirred refluxing mixture of2-fluoro-4-hydroxybenzonitrile (15.0 g, 0.11 mol) and potassiumcarbonate (46.0 g, 0.33 mol) in acetone (300 ml). The stirred mixturewas heated under reflux for 21 h (glc analysis revealed absence ofstarting material). The product was extracted into ether (twice), andthe combined ethereal extracts were washed with water, 5% sodiumhydroxide, water and dried (MgSO₄). The solvent and the excess of1-bromobutane were removed in vacuo to yield a pale-orange solid.

Yield: 19.5 g, 93%.

mp: 25°-26° C.

4-Butoxy-2-fluorobenzoic acid (19) Step 3C

A mixture of concentrated sulphuric acid (190 ml) and water (190 ml) wasadded dropwise to a stirred solution of compound 18 (19.0 g, 0.098 mol)in glacial acetic acid (380 ml). The stirred mixture was heated underreflux for 48 h, cooled in a refrigerator for 24 h and the product wasfiltered off and dried (CaCl₂) in vacuo to give pale-yellow crystals.

Yield: 18.7 g, 90%.

mp: 93°-95° C.

2-Fluoro-4-pent-1'-yn-1'-ylbenzonitrile (20) Step 1C

A solution of n-butyllitium (10.0M in hexane; 10.0 ml, 0.10 mol) wasadded dropwise to a stirred, cooled (<0° C.) solution of pent-1-yne(6.80 g, 0.10 mol) in dry THF* (51 ml) under dry nitrogen. This mixturewas stirred for 10 min and then a solution of zinc chloride* (13.63 g,0.10 mol) in dry THF* (100 ml) was added dropwise at <0° C. The mixturewas stirred at room temperature for 15 min and a solution of compound 3(20.0 g, 0.10 mol) in dry THF* (100 ml) followed by a solution oftetrakis(triphenylphosphine)palladium(O) (2.31 g, 2.0 mmol) in dry THF*(25 ml) were added dropwise at <0° C. The mixture was stirred at roomtemperature for 20 h (glc analysis revealed absence of startingmaterial; 20 h was probably longer than required for reaction) andpoured into 10% hydrochloric acid (200 ml). The product was extractedinto ether (twice), and the combined ethereal extracts were washed withwater, saturated sodium hydrogen carbonate, water and dried (MgSO₄). Thesolvent was removed in vacuo to afford an orange liquid which wasdistilled to yield a pale-yellow oil.

Yield: 17.6 g, 94%.

bp: 100°-105° C. at 0.1 mm Hg.

Zinc chloride (ACS reagent) was dried overnight at 100° C.

2-Fluoro-4-pentylbenzonitrile (21) Step 2C

A stirred mixture of compound 20 (16.0 g, 0.085 mol) and 5%palladium-on-charcoal (4.00 g) in ethanol (100 ml) was hydrogenated atatmospheric pressure for 8 h (glc analysis revealed absence of startingmaterial). The catalyst was filtered off and the solvent was removed invacuo to yield a pale-orange semi-solid.

Yield: 15.35 g, 95%.

mp: not recorded, probably around 20° C.

2-Fluoro-4-pentylbenzoic acid (22) Step 3C

A mixture of concentrated sulphuric acid (150 ml) and water (150 ml) wasadded dropwise to a stirred solution of compound 21 (15.0 g, 0.078 mol)in glacial acetic acid (300 ml). The stirred mixture was heated underreflux for 48 h, cooled in a refrigerator overnight and the product wasfiltered off. The product was dissolved in ether, extracted into 10%sodium hydroxide, which was then acidified with 36% hydrochloric acid,washed with ether (twice), and the combined ethereal phases were washedwith water and dried (MgSO₄). The solvent was removed in vacuo to give abrown crystalline solid.

Yield: 11.5 g, 70%.

mp: 90°-91° C.

4-Butoxybenzoic acid (23) Step 5C

1-Bromobutane (22.6 g, 0.165 mol) was added dropwise at room temperatureto a stirred mixture of 4-hydroxybenzoic acid (15.0 g, 0.11 mol) inethanol (60 ml) and sodium hydroxide (10.60 g) in the minimum amount ofwater. The stirred mixture was heated under reflux overnight (tlcanalysis revealed a complete reaction), and then the ethanol wasdistilled off and an equal volume of water was added. The mixture wasboiled to give a solution, cooled, washed with ether and acidified with36% hydrochloric acid. The cooled mixture was filtered, and the productwas washed with water to give a colourless solid (13.9 g) which wasrecrystallised from ethanol to give colourless crystals.

Yield: 12.1 g, 57%.

mp: 149°-150° C.

Although illustrated in examples 1, 2, 3 and 4 above, and in FIGS. 1, 2,3 and 4 for the preparation of the n-pentyl and n-butyloxy substitutedacids, schemes B and C were found to be equally suitable for thepreparation of all corresponding C₁ -C₁₂ alkyl and alkoxy substitutedcompounds.

EXAMPLE 5

Using schemes E and F shown in FIGS. 5 and 6, the steps of which arelisted below, the compounds of formula I listed in tables 4 and 5 wereprepared. ##STR39##

In FIGS. 5 and 6, X is alkyl or alkoxy.

The experimental method was as follows:

A solution of dicyclohexylcarbodiimide (DCC) (1.15 mol equivalent) indry dichloromethane (50 ml) was added dropwise to a stirred solution ofthe appropriate carboxylic acid (1 mol equivalent), the appropriatephenol (1 mol equivalent) and 4-(N-pyrrolidino)pyridine (0.30 molequivalent) in dry dichloromethane (55 ml) at room temperature. Themixture was stirred at room temperature overnight and theN,N'-dicyclohexylurea (DCU) was filtered off. The filtrate was washedsuccessively with water, 5% acetic acid, water and dried (MgSO₄). Thesolvent was removed in vacuo and the residue was purified by columnchromatography (silica gel/dichloromethane) to give a solid which wasrecrystallised from ethanol (with charcoal decolourisation whererequired) to yield colourless crystals.

A number of compounds of formula I were prepared, and are listed,together with their liquid crystal properties, in tables 4 and 5 below.The preparative routes which were employed to make the phenol andbenzoic acid which were esterified using the DCC mediated method arealso indicated in tables 4 and 5. `Known` in table 4 and 5 indicatesthat commercially available or known acids or phenols were used.

                  TABLE 4                                                         ______________________________________                                                       Acid    Phenol                                                                              K-N                                              Structure                                                                            R       Route   Route (°C.)                                                                       K-I (°C.)                                                                     N-I (°C.)                     ______________________________________                                        1.1    C.sub.5 H.sub.11                                                                      C       D     --   29.5   (-8.0)                               1.1    C.sub.4 H.sub.9 O                                                                     C       D     --   71.5   (7.5)                                1.2    C.sub.5 H.sub.11                                                                      C       Known --   65.5   (32.0)                               1.2    C.sub.4 H.sub.9 O                                                                     C       Known --   90.0   (61.0)                               1.3    C.sub.5 H.sub.11                                                                      C       Known --   39.5   (-3.0)                               1.3    C.sub.4 H.sub.9 O                                                                     C       Known --   55.0   (21.5)                               1.4    C.sub.5 H.sub.11                                                                      C       D     --   55.0   (-20.5)                              1.4    C.sub.4 H.sub.9 O                                                                     C       D     --   68.5   (60)                                 1.5    C.sub.5 H.sub.11                                                                      B       Known --   74.5   [-18.0].sup.(i)                      1.5    C.sub.4 H.sub.9 O                                                                     B       Known --   101.0  [25.0].sup.(iii)                     1.6    C.sub.5 H.sub.11                                                                      B       Known --   32.0   [-45.0].sup.(ii)                     1.6    C.sub.4 H.sub.9 O                                                                     B       Known --   56.0   (-1.5)                               1.7    C.sub.5 H.sub.11                                                                      B       D     --   36.5   (-67)                                1.7    C.sub.4 H.sub.9 O                                                                     B       D     --   63.0   (-31.5)                              ______________________________________                                         () represents monotropic transitions.                                    

                  TABLE 5                                                         ______________________________________                                                       Acid    Phenol                                                                              K-N                                              Structure                                                                            R       Route   Route (°C.)                                                                       K-I (°C.)                                                                     N-I (°C.)                     ______________________________________                                        1.8    C.sub.5 H.sub.11                                                                      Known   A      96.5                                                                              --     150.5                                1.8    C.sub.4 H.sub.9 O                                                                     Known   A     114.5                                                                              --     195.0                                1.9    C.sub.5 H.sub.11                                                                      C       Known 103.0                                                                              --     225.0                                1.9    C.sub.4 H.sub.9 O                                                                     C       Known  94.0                                                                              --     259.5                                1.10   C.sub.5 H.sub.11                                                                      C       A     112.5                                                                              --     187.5                                1.10   C.sub.4 H.sub.9 O                                                                     C       A     115.0                                                                              --     219.5                                1.11   C.sub.5 H.sub.11                                                                      C       A      98.0                                                                              --     143.5                                1.11   C.sub.4 H.sub.9 O                                                                     C       A     104.0                                                                              --     184.0                                1.12   C.sub.5 H.sub.11                                                                      B       Known 109.5                                                                              --     191.0                                1.12   C.sub.4 H.sub.9 O                                                                     B       Known  94.0                                                                              --     223.0                                1.13   C.sub.5 H.sub.11                                                                      B       A     116.5                                                                              --     158.0                                1.13   C.sub.4 H.sub.9 O                                                                     B       A     103.0                                                                              --     194.0                                1.14   C.sub.5 H.sub.11                                                                      B       A      89.0                                                                              --     121.0                                1.14   C.sub.4 H.sub.9                                                                       B       A     102.0                                                                              --     161.5                                ______________________________________                                    

In Table 4, (i), (ii), (iii) indicate that the transitions observed werevirtual, and were observed in 14, 13 and 21 wt % mixtures of thecompounds in the known nematic liquid crystal mixture E7 which has acomposition: ##STR40##

In table 6 below properties of various compounds of formula I having thestructures indicated (see table 1) are listed. The values of viscosity(η cps), ε⊥ and Δε are extrapolated from a 10 weight % solution of thecompound in a 1:1:1 by weight mixture of compounds of formula 3.8 (seetable 3) having the R³ :R⁴ combinations n-C₃ H₇ :C₂ H₅, n-C₃ H₇ :n-C₅H₁₁, n-C₅ H₁₁ : C₂ H₅. These solutions are examples of liguid crystalmixtures of the invention.

                  TABLE 6                                                         ______________________________________                                        Structure                                                                              R          η  Dipole  ε⊥                                                                    Δε                      ______________________________________                                        1.1      n-C.sub.5 H.sub.11                                                                              7.2     9.7  61                                    1.2      "                 6.4     8.6  39                                    1.3      "                 7.1     10.4 50.6                                  1.4      "                 7.7     10.0 64.9                                  1.5      "                 6.8     7.9  36.3                                  1.6      "           86    7.4     19.0 40.0                                  1.7      "                 8.0     11.9 59.3                                  1.9      "          420    7.5     10.0 54                                    1.1      n-C.sub.4 H.sub.9 O                                                                             7.8     10.2 66.8                                  1.2      "                 6.9     7.5  42.6                                  1.3      "                 7.5     10.7 43.8                                  1.4      "                 8.0     11.3 69.5                                  1.5      "                         7.6  44.5                                  1.6      "                 7.9     9.9  54.2                                  1.7      "                 8.3     10.7 58.5                                  ______________________________________                                    

Referring to FIG. 7, a liquid crystal electro optical display devicecomprises a lyer of a liquid crystal material 31 between a front glasssupport plate 32 and a back glass support plate 33. The front glassplate 32 is coated on the inner surface thereof with a tansparentconducting layer 34 e.g. tin oxide, to form an electrode. The back glassplate 33 is also coated on the inside surface thereof with a conductivelayer 35. If light is to be transmitted through the device the backelectrode 15 and the back plate 33 are also transparent and made of thesame materials as the front plate and electrode 32 and 34. If the deviceis to reflect light the back electrode 35 may be made reflective, e.g.of aluminium. The front and back electrodes 34, 35 are coated with atransparent aligning layer 36, 37 e.g. of poly-vinyl alcohol which haveeach been rubbed prior to assembly of the device in a single rubbingdirection. On assembly the rubbing directions of the layers 36, 37 arearranged orthogonal to each other. In operation the electrodes 34, 35are connected to a source of voltage (not shown).

A suitable liquid crystal material 32 is a 14, 13, or 21 weight %mixture of one of the compounds: ##STR41## in the material E7 referredto above.

We claim:
 1. A compound suitable for inclusion in a nematic liquidcrystal device having the formula II: ##STR42## in which R is R¹ or R₁O, wherein R¹ is C₁ -C₁₂ alkyl, a, b, c and d are independently 0 or 1,provided (a+b+c+d) is not zero, and excluding the case where a and b are0 and one of c or d is one.
 2. A compound according to claim 1 havingthe formula: ##STR43##
 3. A compound according to claim 1 having theformula: ##STR44##
 4. A compound according to claim 1 having theformula: ##STR45##
 5. A compound according to claim 1 having theformula: ##STR46##
 6. A compound according to claim 1 having theformula: ##STR47##
 7. A compound according to claim 1 having theformula: ##STR48##
 8. A compound according to claim 1 having theformula: ##STR49##